This invention relates to a composition and a method for lubricating a steel belt continuously variable transmission (CVT).
The continuing pursuit of more fuel efficient motor vehicles has led to the development of continuously variable transmissions by a number of manufacturers. The major difference between a continuously variable transmission and a conventional automatic transmission is that automatic transmissions use planetary gear sets to accomplish speed changes, whereas a continuously variable transmission uses pulleys and a belt to change speed. A conventional automatic transmission normally has 3, 4 or 5 fixed reduction ratios or xe2x80x9cspeedsxe2x80x9d, e.g., a 5-speed automatic transmission. The operating system of the transmission selects the appropriate reduction ratio, or speed, based on engine rpm, ground speed and throttle position. In a continuously variable transmission an almost infinite number of reduction ratios, within fixed limits, can be achieved by changing the relative radius of travel of the driving belt on the driving and driven pulleys.
The critical mechanism in the CVT is the variator. The variator is composed of two steel pulleys and a steel belt. The pulleys can be opened and closed thereby allowing the belt to travel at different radiuses. When the driving pulley is fully opened (small radius of belt travel) and the driven pulley is fully closed (large radius of belt travel) very high reduction ratios are achieved (yielding low ground speeds). Conversely when the driving pulley is fully closed (large radius of belt travel) and the driven pulley is fully opened (small radius of belt travel) increases in output speed over input speed are achieved. (yielding high ground speeds).
The novelty of this design is that the belt is made of steel. Two types of CVT transmissions exist. In one design, the belt is xe2x80x9cpushedxe2x80x9d or compressed to transmit power, and in the other the belt is pulled, as is more common with a V-belt. Since steel belts are used in contact with steel pulleys, the lubrication requirements are identical for both design types.
There are two critical requirements for the lubricants used in CVT transmissions: (1) control of wear and (2) control of friction. Since steel-on-steel coefficients of friction tend to be very low, e.g., 0.03 to 0.2, extremely high closing forces are applied to the pulley sides to keep the belt from slipping. Any slippage of the belt causes catastrophic wear which quickly leads to failure. The pulleys are made to exacting limits and have a precise surface finish to allow optimum operation. No wear of these surfaces can be allowed. Therefore, an appropriate lubricant must have excellent wear control. The frictional characteristics of the belt-pulley. interface are also critical. The friction must be very high to prevent slippage of the belt during transmission of high torque from the engine to the drive wheels. Too high a static coefficient of friction, however, can cause xe2x80x9cstickingxe2x80x9d of the belt which will lead to oscillation and audible noise in the passenger compartment of the vehicle. This xe2x80x9cwhistlingxe2x80x9d of the belt is highly undesirable.
We have now found a unique combination of antiwear additives and friction modifiers that solve the difficult lubrication problems created by the steel-on-steel pulley system used in a continuously variable transmission.
This invention relates to a composition and a method of lubricating a continuously variable transmission comprising:
(1) a major amount of a lubricating oil; and
(2) an effective amount of a performance enhancing additive combination comprising:
(a) an organic phosphite;
(b) an amine salt of an organic phosphate; and
(c) one or more friction modifiers chosen from:
(1) selected amides,
(2) succinimides, and
(3) ethoxylated amines.
Lubricating the variator system of a CVT is not a simple matter. It presents a unique problem of controlling wear and friction to very exacting limits. The antiwear agents must be carefully selected to provide excellent wear control and yet not interfere with the friction modifiers. The friction modifiers must be selected so as to provide very precise control of the steel-on-steel friction and not interfere with the control of wear.
Lubricating oils useful in this invention are derived from natural lubricating oils, synthetic lubricating oils, and mixtures thereof. In general, both the natural and synthetic lubricating oil will each have a kinematic viscosity ranging from about 1 to about 100 mm2/s (cSt) at 100xc2x0 C., although typical applications will require the lubricating oil or lubricating oil mixture to have a viscosity ranging from about 2 to about 8 mm2/s (cSt) at 100xc2x0 C.
Natural lubricating oils include animal oils, vegetable oils (e.g., castor oil and lard oil), petroleum oils, mineral oils, and oils derived from coal or shale. The preferred natural lubricating oil is mineral oil.
Suitable mineral oils include all common mineral oil basestocks. This includes oils that are naphthenic or paraffinic in chemical structure. Oils that are refined by conventional methodology using acid, alkali, and clay or other agents such as aluminum chloride, or they may be extracted oils produced, for example, by solvent extraction with solvents such as phenol, sulfur dioxide, furfural, dichlorodiethyl ether, etc. They may be hydrotreated or hydrofined, dewaxed by chilling or catalytic dewaxing processes, or hydrocracked. The mineral oil may be produced from natural crude sources or be composed of isomerized wax materials or residues of other refining processes.
Typically the mineral oils will have kinematic viscosities of from 2.0 mm2/s (cSt) to 8.0 mm2/s (cSt) at 100xc2x0 C. The preferred mineral oils have kinematic viscosities of from 2 to 6 mm2/s (cSt), and most preferred are those mineral oils with viscosities of 3 to 5 mm2/s (cSt) at 100xc2x0 C.
Synthetic lubricating oils include hydrocarbon oils and halo-substituted hydrocarbon oils such as oligomerized, polymerized, and interpolymerized olefins [e.g., polybutylenes, polypropylenes, propylene, isobutylene copolymers, chlorinated polylactenes, poly(1-hexenes), poly(1-octenes), poly-(1-decenes), etc., and mixtures thereof]; alkylbenzenes [e.g., dodecyl-benzenes, tetradecylbenzenes, dinonyl-benzenes, di(2-ethylhexyl)benzene, etc.]; polyphenyls [e.g., biphenyls, terphenyls, alkylated polyphenyls, etc.]; and alkylated diphenyl ethers, alkylated diphenyl sulfides, as well as their derivatives, analogs, and homologs thereof, and the like. The preferred oils from this class of synthetic oils are oligomers of xcex1-olefins, particularly oligomers of 1-decene.
Synthetic lubricating oils also include alkylene oxide polymers, interpolymers, copolymers, and derivatives thereof where the terminal hydroxyl groups have been modified by esterification, etherification, etc. This class of synthetic oils is exemplified by: polyoxyalkylene polymers prepared by polymerization of ethylene oxide or propylene oxide; the alkyl and aryl ethers of these polyoxyalkylene polymers (e.g., methyl-polyisopropylene glycol ether having an average molecular weight of 1000, diphenyl ether of polypropylene glycol having a molecular weight of 1000 to 1500); and mono- and poly-carboxylic esters thereof (e.g., the acetic acid esters, mixed C3-C8 fatty acid esters, and C12 oxo acid diester of tetraethylene glycol).
Another suitable class of synthetic lubricating oils comprises the esters of dicarboxylic acids (e.g., phthalic acid, succinic acid, alkyl succinic acids and alkenyl succinic acids, maleic acid, azelaic acid, suberic acid, sebasic acid, fumaric acid, adipic acid, linoleic acid dimer, malonic acid, alkylmalonic acids, alkenyl malonic acids, etc.) with a variety of alcohols (e.g., butyl alcohol, hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, ethylene glycol, diethylene glycol monoethers, propylene glycol, etc.). Specific examples of these esters include dibutyl adipate, di(2-ethylhexyl) sebacate, di-n-hexyl fumarate, dioctyl sebacate, diisooctyl azelate, diisodecyl azelate, dioctyl phthalate, didecyl phthalate, dieicosyl sebacate, the 2-ethylhexyl diester of linoleic acid dimer, and the complex ester formed by reacting one mole of sebasic acid with two moles of tetraethylene glycol and two moles of 2-ethyl-hexanoic acid, and the like. A preferred type of oil from this class of synthetic oils are adipates of C4 to C12 alcohols.
Esters useful as synthetic lubricating oils also include those made from C5 to C12 monocarboxylic acids and polyols and polyol ethers such as neopentyl glycol, trimethylolpropane pentaerythritol, dipentaerythritol, tripentaerythritol, and the like.
Silicon-based oils (such as the polyalkyl-, polyaryl-, polyalkoxy-, or polyaryloxy-siloxane oils and silicate oils) comprise another useful class of synthetic lubricating oils. These oils include tetraethyl silicate, tetraisopropyl silicate, tetra-(2-ethylhexyl) silicate, tetra-(4-methyl-2-ethylhexyl) silicate, tetra-(p-tert-butylphenyl) silicate, hexa-(4-methyl-2-pentoxy)-disiloxane, poly(methyl)-siloxanes and poly(methylphenyl) siloxanes, and the like. Other synthetic lubricating oils include liquid esters of phosphorus-containing acids (e.g., tricresyl phosphate, trioctyl phosphate, and diethyl ester of decylphosphonic acid), polymeric tetrahydrofurans, poly-xcex1-olefins, and the like.
The lubricating oils may be derived from refined, rerefined oils, or mixtures thereof. Unrefined oils are obtained directly from a natural source or synthetic source (e.g., coal, shale, or tar sands bitumen) without further purification or treatment. Examples of unrefined oils include a shale oil obtained directly from a retorting operation, a petroleum oil obtained directly from distillation, or an ester oil obtained directly from an esterification process, each of which is then used without further treatment. Refined oils are similar to the unrefined oils except that refined oils have been treated in one or more purification steps to improve one or more properties. Suitable purification techniques include distillation, hydrotreating, dewaxing, solvent extraction, acid or base extraction, filtration, and percolation, all of which are known to those skilled in the art. Rerefined oils are obtained by treating used oils in processes similar to those used to obtain the refined oils. These rerefined oils are also known as reclaimed or reprocessed oils and are often additionally processed by techniques for removal of spent additives and oil breakdown products.
When the lubricating oil is a mixture of natural and synthetic lubricating oils (i.e., partially synthetic), the choice of the partial synthetic oil components may widely vary, however, particularly useful combinations are comprised of mineral oils and poly-xcex1-olefins (PAO), particularly oligomers of 1-decene.
The lubricating oil is combined with an additive formulation. One component of the additive system of the current invention is an organic phosphite. The organic phosphites useful in this invention are the mono-, di- and tri-hydrocarbyl phosphites having the structures I and IA, where structures I and IA are represented by: 
where R is hydrocarbyl and R1 is hydrocarbyl or hydrogen; 
where R, Rxe2x80x2, and Rxe2x80x3 are hydrocarbyl. As used herein, the term xe2x80x9chydrocarbylxe2x80x9d denotes a group having a carbon atom directly attached to the remainder of the molecule and having predominantly hydrocarbon character within the context of this invention. Such groups include the following: (1) hydrocarbon groups; that is, aliphatic, alicyclic (e.g., cycloalkyl or cycloalkenyl), aromatic groups, alkaryl groups, and the like, as well as cyclic groups wherein the ring is completed through another portion of the molecule; (2) substituted hydrocarbon groups; that is, groups containing non-hydrocarbon substituents which in the context of this invention, do not alter the predominantly hydrocarbon nature of the group. Those skilled in the art will be aware of suitable substituents. Examples include, halo, hydroxy, nitro, cyano, alkoxy, acyl, etc.; (3) hetero groups; that is, groups which while predominantly hydrocarbon in character within the context of this invention, contain atoms of other than carbon in a chain or ring otherwise composed of carbon atoms. Suitable hetero atoms will be apparent to those skilled in the art and include, for example, nitrogen, oxygen and sulfur.
In structures I and IA, when R, R1, Rxe2x80x2, or Rxe2x80x3 is alkyl, the alkyl groups are C4 to C20, preferably C6 to C18, most preferably C8 to C16. When R, R1, Rxe2x80x2, or Rxe2x80x3 is aryl, the aryl groups consist of from 6 to 30 carbon atoms, preferably C6 to C12, and contain at least one unsaturated xe2x80x9caromaticxe2x80x9d ring structure. Such groups are known to those skilled in the art. Examples include methyl, ethyl, octyl, decyl, octadecyl, cyclohexyl and phenyl, etc. R, R1, Rxe2x80x2, and Rxe2x80x3 can also vary independently. As stated, R, R1, Rxe2x80x2, and Rxe2x80x3 can be alkyl, aryl, may be linear or branched, and the aryl groups may be phenyl or substituted phenyl. The R, R1, Rxe2x80x2, and Rxe2x80x3 groups may be saturated or unsaturated, and they may contain hetero atoms such as S, N or O. The preferred materials are the dialkyl phosphites (structure I). The R and R1 groups are preferably linear alkyl groups such as butyl, octyl, decyl, hexadecyl and octadecyl. The most preferred are decyl, undecyl, 3-thiaundecyl, pentadecyl and 3-thiapentadecyl.
Phosphites of structures I and IA may be used individually or in mixtures.
Another embodiment of this invention is the use of the mixed alkyl phosphites described in U.S. Pat. Nos. 5,185,090 and 5,242,612.
While any effective amount of the organic phosphite may be used to achieve the benefits of the invention, typically these effective amounts will be from 0.01 to 5.0 mass percent in the finished fluid. Preferably the treat rate in the fluid will be from 0.2% to 3.0% and most preferred is 0.3% to 1.0%.